This book is composed of eight chapters and begins with the principles of the absolute measurement of radiant power. The subsequent chapters provide the criteria associated with reflectance and transmittance of optical radiation and the parameters used to characterize the performance of radiation detectors. A chapter presents an analysis of the temperature distribution in a detector element. This topic is followed by discussions of the environmental and instrumental corrections in absolute radiometry. The final chapters deal with the alternative optical power scales and direct current substitution methods used in other fields of metrology.